It is known that (±)-3a,6,6,9a-tetramethyl-(3aα,5aβ,9aα,9bβ)-decahydronaphtho[2,1-b]furan-2(1H)-one represented by the below-mentioned general formula (VI) (hereinafter occasionally referred to merely as “(±)-sclareolide”) is one of diastereomers of (±)-3a,6,6,9a-tetramethyldecahydronaphtho[2,1-b]furan-2(1H)-ones represented by the below-mentioned general formula (III), and is a useful compound as a precursor, etc., for (±)-3a,6,6,9a-tetramethyl-(3aα, 5aβ,9aα,9bβ)-dodecahydronaphtho[2,1-b]furan represented by the below-mentioned general formula (VII) (hereinafter occasionally referred to merely as “(±)-ambroxan”) which is an important amber-like perfume material having, in particular, an excellent aromatizing property and an excellent fragrance persisting or lingering property among (±)-3a,6,6,9a-tetramethyldodecahydronaphtho[2,1-b]furans represented by the below-mentioned general formula (V).

The ambroxan inherently means (−)-ambroxan which is an optically active substance derived from natural substances. There are disclosed many processes for producing the ambroxan from (−)-sclareol as a starting material which is an extract from clary sage as a natural plant, via (+)-sclareolide (for example, refer to Non-Patent Document 1), and these processes have been practically used in industrial applications. However, these conventional production processes have problems such as less amounts of the natural raw materials supplied and unstable supply thereof. In addition, in the processes, since an oxidant such as chromic acid and permanganates is used in an oxidative decomposition step thereof, there also tends to occur such a problem that the processes have a large burden on environments.
For this reason, it has been demanded to develop an inexpensive process for producing the (±)-sclareolide and (±)-ambroxan from alternative petrochemical raw materials.
To meet the above demand, there is disclosed a process for producing (±)-ambroxan via (±)-sclareolide which includes, for example, six steps as shown in the following reaction formula (A) using farnesol or nerolidol as a starting material (for example, refer to Patent Document 1).

According to the above process, although the inexpensive raw material is converted into homofarnesylic acid with a relatively high yield, there tends to occur such a problem that the process is not fully suitable for industrial-scale production of the aimed compounds because the reagents having an extremely strong toxicity or corrosiveness such as potassium cyanide and phosphorus tribromide are used in an equimolar amount or more based on the raw material.
Further, there are many other reports describing a process for producing the compounds represented by the above general formula (III) by cyclizing homofarnesylic acid. From these reports, it is known that the diastereo-selectivity to the (±)-sclareolide largely varies depending upon kind of an acid agent and reaction conditions such as reaction temperature as used in the reaction. However, in order to produce, as a main reaction product, the (±)-sclareolide which is a more suitable diastereomer, it is advantageous to conduct the reaction in the presence of a very strong acid agent or under an extremely low temperature. Therefore, the above process is not fully suitable for industrial-scale production of the aimed compounds.
In addition, there are also many reports concerning the process for producing the (±)-ambroxan without via the (±)-sclareolide. In particular, there are known the processes represented by the following two reaction formulae (B) and (C) in which the compounds represented by the general formula (V) are obtained from (+)-(E)-nerolidol through the three steps, and from dihydro-β-ionone through the four steps, respectively (for example, refer to Non-Patent Document 2).

These processes have such an advantage that the number of steps thereof is short as compared to those of the conventional processes for production of (±)-ambroxan. On the other hand, in these processes, a strong reducing agent (such as lithium triethyl boron hydride) having a risk of ignition due to violent reaction with water such as moisture in air must be used in the step of reducing the (3E,7E)-homofarnesylic acid dimethyl amide or (E)-β-monocyclo-homofarnesylic acid dimethyl amide into the respective corresponding alcohol compounds. As a result, these processes are not fully suitable for industrial-scale production of the aimed compounds.
Patent Document 1: DE 3240054
Non-Patent Document 1: “Tetrahedron”, Vol. 43, p. 1871, 1987
Non-Patent Document 2: “Journal of Organic Chemistry”, Vol. 61, p. 2215, 1996